It depends on what is wrong with your knee. If it is more the joint or knee cap. Or if it is more muscle imbalance(quads tend to be stronger than hamstrings) that causes stress to the knee. Your best bet would be to consult with a PT to assess the root of the issue and get ideas from there.

I agree it depends what's wrong with your knee, I would also check with your PT.

However, I am 3 months post acl surgery and your more than welcome to follow my workouts which are designed to strengthen the muscles around the knee whilst being low impact. Also I am doing other body parts without stress on the knee. I.e. No burpees, running, etc for now.